Jan,
> In the case that a table constraint is a referential constraint,
> the table is referred to as the referencing table. The referenced
> columns of a referential constraint shall be the unique columns of
> some unique constraint of the referenced table.
Missed that one. Interesting. AFAIK, the uniqueness of referenced columns is
NOT a requirement of Relaitonal Algebra. So why does SQL require it?
Maybe I'll ask Joe Celko after he finishes moving to Austin.
I have my own issue that forced me to use triggers. Given:
table users (namelogin PKstatusetc. )
table status (statusrelationlabeldefinitionPK status, relation )
the relationship is:
users.status = status.status AND status.relation = 'users';
This is a mathematically definable constraint, but there is no way in standard
SQL to create an FK for it. This is one of the places I point to whenever
we have the "SQL is imperfectly relational" discussion.
--
Josh Berkus
Aglio Database Solutions
San Francisco
